In recent years, Multi-Input/Multi-Output (MIMO) communication has been drawing an attention as a technology for enabling a communication of massive data such as images. In the MIMO communication, different transmitted data (sub streams) are respectively transmitted from a plurality of antennas in a transmitter side, and a plurality of the transmitted data that are mixed in a propagation path are separated into the respective original transmitted data in a receiver side by using a propagation path estimate (see, for example, Japanese Patent Laid-Open No.2002-44051 (FIG. 4)).
In an actual operation, in the MIMO communication, signal transmitted from a transmission apparatus is received with the antennas, the number of which is equal to or larger than the number of the transmission apparatuses, and the propagation path characteristics between antennas are estimated based on pilot signals, which are respectively inserted into signals received with the respective antennas. This estimated propagation path characteristic H is represented by a matrix of 2×2, where, for example, number of the transmitting antennas is two and number of the receiving antenna is two. In the MIMO communication, based on an inverse matrix of the obtained propagation path characteristic H and received signals obtained with respective receiving antennas, transmission signals (sub streams) transmitted by respective transmitting antennas are found.
With reference to FIG. 1A, principle of the MIMO communication will be described for a case where the number of antennas of a transmitter 10 and that of a receiver 20 are respectively two. Here, signals transmitted via antennas 11 and 12 of the transmitter 10 are represented as TX1 and TX2, respectively, and signals received via antennas 21, 22 of the receiver 20 are represented as RX1 and RX2, respectively.
With this assumption, the received signals (RX1, RX2) can be expressed with (equation 1) shown in FIG. 1B. Here, A represents a propagation path characteristic between the transmitting antenna 11 and the receiving antenna 21, B represents a propagation path characteristic between the transmitting antenna 12 and the receiving antenna 21, C represents a propagation path characteristic between the transmitting antenna 11 and the receiving antenna 22, and D represents a propagation path characteristic between the transmitting antenna 12 and the receiving antenna 22.
Thus, as for the antenna 21 and 22 of the receiver 20, the signal is received in a form of a mixed combination of TX1 and TX2, as expressed in (equation 1). In order to separate TX1 and TX2, for example, either one of TX1 and TX2 is defined as a desired signal component and the other is defined as an interference signal component, and the interference signal component should be compensated.
In order to remove (compensate) the interference signal component stated above and to obtain the transmission signal (TX1, TX2) from the received signal, an inverse matrix of a matrix consisting of these four propagation path characteristics A, B, C and D is found as expressed in (equation 2). Therefore, the transmitter 10 transmits the signal containing a known signal for propagation path estimation (pilot signal, for example) inserted in the transmission signal, and the receiver 20 conducts a propagation path estimation based on this known signal to obtain the propagation path characteristics A, B, C and D, thereby finding the above-described inverse matrix.
Procedures for actually finding the transmission signal (TX1, TX2) from the received signal (RX1, RX2) includes: a Zero-Forcing (ZF) arithmetic operation for separating a sub stream (respective data) by using only an inverse matrix arithmetic operation presented by (equation 2), or Minimum Mean Square Error (MMSE) arithmetic operation for separating so as to minimize an error, and the like.
As such, in the MIMO communication, a plurality of signals, which have been transmitted at the same time at the same frequency, can be theoretically separated respectively in the receiver, and thus the communication at higher rate with higher capacity becomes possible. However, since there is an influence such as an inter-code interference due to a noise or a multipath in an actual apparatus, and/or since there is also a quantization error or the like in an actual circuitry, an interference compensation error is generated in the process of compensating for an interference signal component from the transmission signal, and there is a problem that error rate characteristic in the receiving side significantly deteriorates when this error is larger. In addition, depending on the propagation environment, a value of a determinant |AD-BC| of the inverse matrix represented in FIG. 1B (equation 2) may be closer to zero, and since the conventional apparatus attempts to compensate for the interference signal component even in such situation, another problem occurs that an interference compensation error in the separated desired signal becomes greater, thereby significantly deteriorating the error rate in the receiving side.